1. Field of the Invention
The present invention relates to a one-component type polyurethane adhesive, more particularly, to a one-component type polyurethane adhesive which comprises a polyurethane precursor as a main component having uretdione groups and active hydrogen groups at a prescribed ratio and which cures by chain extension and crosslinking on simple heating without evolution of a free polyisocyanate monomer, and is excellent in operability and safety, and the use thereof.
2. Related Background Art
Polyurethane type adhesives used for general purpose are mostly of solvent types from the standpoint of ease of operation.
Polyurethane adhesives are roughly classified into the following three classes according to the curing mechanism.
The first class of the adhesives are two-component type polyurethane adhesives, in which a first component of a polyisocyanate and/or an isocyanate-terminated prepolymer with a second component of a low-molecular-weight polyol and/or urethane-modified polyol having molecular weight of less than 10,000, and are mixed immediately before application, and applied to a base material (adherend) to be cured.
The second class of the adhesives are moisture-curing type polyurethane adhesives, which contain an isocyanate-terminated prepolymer and cure by reaction with active hydrogen groups of a base material or moisture in the air. This group includes non-solvent type reactive hot-melt adhesives which are melted at a temperature of 100.degree. C. or higher, applied to a base material and cooled to solidify to exhibit initial adhesive force, and later react with active hydrogens on the base material or moisture in air to be cured by polymerization and crosslinking.
The third class of adhesives are one-can lacquer type polyurethane adhesives, which are a solution or dispersion of a high-molecular-weight thermoplastic polyurethane resin in a solvent or water, and the solution is applied to a base material. The high-molecular-weight polyurethane resin having high cohesive energy forms adhesive layer to exhibit adhesive force by simple evaporation of the solvent or the water.
The respective classes of adhesives have advantages and disadvantages as described below.
The two-component type polyurethane adhesive forms an adhesive layer which has generally a crosslinked structure and is superior in heat-resistance and durability. However, its pot life is limited because the liquid formulation of the two components becomes viscous due to the reaction of the isocyanate group with the hydroxy group in the system, and gels finally.
The moisture-curing type polyurethane adhesive forms an adhesive layer which gives final adhesive force weaker than a two-component type polyurethane adhesive but are superior in heat resistance and other properties. Since the adhesive force is usually caused by the reaction with moisture on the base material or in air, the adhesive has disadvantages that the reaction proceeds slowly and the initial adhesive force is exhibited slowly, and the adhesiveness depends on external conditions such as low temperature and low humidity in winter, and high temperature and high humidity in summer.
The above two classes of adhesives, namely the two-component type polyurethane adhesives and the moisture-curing type polyurethane adhesives, contain a free polyisocyanate monomer in a slight amount in the system. The free polyisocyanate monomer tends to aggravate working environment depending on the conditions of application of the adhesive, and frequently necessitates local ventilation. The one-can lacquer type polyurethane adhesive, which is a solution of high-molecular-weight polyurethane resin such as a thermoplastic polyurethane solution, has a semipermanent pot life unless the solvent is not evaporated, and is easily handled. This type of adhesive, however, forms an adhesive layer which has thermoplasticity and a low melting point due to no crosslinked structure, and therefore the heat-resistance is low since the adhesion strength thereof falls at a temperature exceeding the softening temperature.
Many proposals have been presented regarding improvement of polyurethane adhesives. For example, regarding the one component lacquer type polyurethane adhesives, a proposal is presented that a polyisocyanate curing agent is added thereto (two-pack type). Regarding the two component type polyurethane adhesive, a composition of a blocked isocyanate is proposed which is seemingly a one component system and comprises an isocyanate curing agent blocked with a blocking agent that generates isocyanate groups by dissociation and evaporation of the blocking agent in heating higher than 120.degree. C. and a low-molecular-weight polyol and/or a urethane-modified polyol. Use of this composition still involves problems of adhesive layer foaming caused by the blocking agent, pollution caused by evaporation of the blocking agent, drop of adhesive force resulting from residual blocking agent, and so forth. Therefore, this composition is rarely used for adhesives, although it is used as a one-pack paint system in some of paint application fields. Regarding moisture-curing type polyurethane adhesive, there is proposed a formulation which is less affected by external conditions as the results of incorporation of a hygroscopic composition or a catalyst; a seemingly one component system which contains a curing agent such as kerimine compound (Schiff base) and aldimine compound to generate an amino group in contact with moisture, and an isocyanate-terminated prepolymer system; and so forth. No complete one component type adhesive has not been obtained which is storable for a long time. Therefore, the development thereof is strongly demanded at the moment.
Furthermore, from the global environmental problems, VOC (Volatile Organic Compound) regulation is being tightened, and movements toward resource conservation, non-pollution, and higher safety are required in the fields of paints, adhesives, and inks. In view of the decreasing pollution, adhesives and paints of high-solid type, powder coating type, water-borne type, and hot-melt type are attracting attention. At present, however, any high-solid type or non-solvent type polyurethane adhesive for general purpose has not been developed which satisfy both the ease of handling and the excellent characteristics of the solvent type polyurethane adhesive containing a large amount of a solvent. Therefore, the development thereof is strongly demanded. Moreover, development of a polyurethane adhesive which satisfies the VOC regulation and contains no free isocyanate monomer is strongly demanded.
On the other hand, uretdione group-containing polyurethane resins are already known (See: Kunststoffhandbuch, Volum VII, pages 17 to 37 (1966). It is also known that a uretdione group-containing compound opens the uretdione group which is a dimer of isocyanate group to regenerate two isocyanate groups (J. H. Saunders, K. C. Frisch: Polyurethanes, Chemistry and technology, Part 1, p. 113 (1962), Interscience Publishers).
Further, uretdione group-containing polyisocyanate compounds and related application techniques are known as shown in U.S. Pat. Nos. 4,044,171, 4,430,474, 4,442,280 and 4,801,663; and Japanese Patent Application Laid-open No. Hei-2-620; and so forth. The techniques relating to toluene diisocyanate dimer have long been known in the field of powder coating and millable rubber hardeners. Uretdione group-containing polyurethane resins of isophorone diisocyanate type and diphenylmethane diisocyanate type have come to be known recently.
The features of the novel polyurethane adhesive of the present invention to meet the above demands are: (1) sufficient adhesiveness to various base materials and excellent film properties owing to high cohesive energy characteristic of conventional polyurethane adhesives, (2) ease of handling in coating and spraying owing to low viscosity which is achieved by two-component type polyurethane adhesives, (3) excellent heat-resistance and excellent durability owing to crosslinking formed by reaction at the time of adhesion, (4) semipermanent storage stability and long pot life which are achievable by lacquer type adhesives and blocked isocyanate systems, (5) no liberation or no evaporation of free polyisocyanate monomer during formulation and heating, and reduction of the amount of a solvent to meet the VOC regulation for environment protection, and (6) high-solid type, non-solvent type, or solid type of adhesive.